Microgravity can disrupt the human ability to coordinate both arms, particularly when each limb must follow a different timing pattern. A parabolic-flight study found that complex two-arm movements became less accurate and less stable in weightlessness, while partial gravity restored some of the lost coordination.
The results suggest that gravity does more than provide physical resistance. It may also serve as an important sensory reference that helps the nervous system organise the timing of coordinated movements. The findings could inform the development of astronaut training and feedback systems for tasks performed in orbit or during future missions to the Moon and Mars.
The research was published in Human Movement Science by Madison M. Weinrich and colleagues from Texas A&M University, California State University San Marcos, Anhembi Morumbi University, KBR and Ohio University.
Testing coordination during parabolic flight
The study involved 12 right-handed adults with an average age of about 40. Ten participants had previously experienced parabolic flight, while the other two had comparable high-performance flight experience.
Experiments were conducted aboard Novespace's Airbus A310 Zero G during the European Space Agency's 82nd parabolic-flight campaign. Participants experienced five gravitational conditions: microgravity at 0 g, partial gravity at 0.25 g, 0.50 g and 0.75 g, and normal Earth gravity at 1 g.
The participants sat in modified aircraft seats with their elbows and forearms supported. Force sensors positioned beneath their wrists measured the pressure produced independently by each arm. This isometric design allowed the researchers to study motor coordination without requiring large arm movements that could be disturbed by the aircraft's motion.
Participants received real-time visual feedback through head-mounted displays. A moving cursor showed the force produced by each arm within a Lissajous pattern, a visual shape that represents the relationship between two repeating signals.
Three different movement patterns
The experiment tested three forms of bimanual coordination:
- 0° in-phase coordination: Both arms produced force simultaneously.
- 180° anti-phase coordination: The arms alternated, with one producing force as the other relaxed.
- 90° coordination: One arm followed the other by one-quarter of a movement cycle.
These patterns have different levels of natural stability. In-phase movement is generally the easiest for the nervous system to maintain. Anti-phase coordination is less stable, while the 90° pattern is particularly difficult because the limbs must maintain an offset that the motor system does not naturally favour.
In coordination dynamics, stable patterns are sometimes described as attractors. When movement becomes disturbed, the system tends to return to these preferred patterns. The 0° pattern is considered the strongest attractor, while 90° coordination behaves more like an unstable state.
For a fair comparison, the researchers analysed the central 10 seconds of every trial, regardless of the total duration of each gravitational phase. They measured phase accuracy, directional bias, timing stability, force variability and the proportion of time participants remained close to the target pattern.
Microgravity had its strongest effect on complex coordination
At 1 g, performance followed the expected hierarchy. The simultaneous 0° pattern was the most stable, the alternating 180° pattern was less stable, and the 90° task produced the greatest overall error and variability.
Removing gravitational load selectively weakened this structure. During the 90° task, absolute coordination error was significantly higher in microgravity than at every partial-gravity level and at 1 g. Participants also spent significantly less time close to the intended 90° relationship in 0 g.
The simpler 0° pattern remained comparatively robust. This indicates that microgravity did not reduce every form of coordination equally. Instead, it exposed the vulnerability of movement patterns that were already difficult for the nervous system to stabilise.
Movements drifted towards the simplest pattern
One of the study's clearest findings was a systematic drift towards in-phase movement. In microgravity, participants attempting both the 90° and 180° tasks tended to move closer to the more stable 0° pattern.
The direction of the measured error differed between the two tasks because they began on opposite sides of the coordination cycle. However, both changes represented the same underlying tendency: when gravitational input was removed, the motor system was increasingly drawn towards simultaneous movement.
The researchers interpret this as evidence that microgravity makes weaker coordination states shallower and easier to disturb, allowing the strongest natural movement pattern to exert greater influence.
Partial gravity restored some stability
Performance improved under partial gravity. In the 90° task, accuracy, directional control, movement timing and time spent near the target were generally better at 0.25 g, 0.50 g and 0.75 g than in microgravity.
This recovery was not perfectly stepwise across every measurement. A higher gravity level did not always produce an equally proportional improvement. Nevertheless, the overall results show that partial gravitational input provided more stable coordination than complete weightlessness, particularly for the most difficult movement pattern.
This distinction is relevant to future exploration environments. The Moon and Mars have different gravitational levels, and neither exactly matches the partial-gravity conditions tested in this experiment. The findings therefore cannot directly predict astronaut performance on either world, but they provide evidence that motor control may differ substantially between microgravity and planetary surfaces.
Timing was affected more consistently than force
The researchers found that gravity's effects were most clearly expressed through movement timing and the relationship between the two limbs. In microgravity, timing became more variable and participants had greater difficulty maintaining the intended phase difference.
Changes in force magnitude were smaller and depended more heavily on the particular task. This suggests that the main challenge was not simply producing enough pressure with each arm. The greater difficulty was correctly organising when each arm should produce that force in relation to the other.
Why gravity may help organise movement
Gravity continuously contributes to posture, muscle tone, balance and information from the vestibular and proprioceptive systems. The brain normally combines these signals with vision to estimate the body's position and organise movement.
In microgravity, that familiar relationship changes. Visual feedback remains available, but the usual gravitational, vestibular and muscular references are reduced or reorganised. The study found that visual Lissajous feedback alone was not sufficient to fully stabilise the more difficult coordination patterns.
The researchers therefore describe gravity as a contextual control parameter: an environmental condition that changes the stability of movement patterns rather than merely adding physical weight to the limbs. The proposed sensory and neurological mechanisms remain interpretations, however, and were not measured directly in the experiment.
Possible implications for astronaut training
Many activities in spacecraft require coordinated use of both hands, including operating controls, handling tools, maintaining equipment and conducting scientific experiments. Tasks in which each hand performs a different action or follows a different rhythm may be especially sensitive to altered gravity.
The authors suggest that training systems could explore additional forms of assistance, including auditory pacing, haptic cues and adaptive visual feedback. Such systems might provide alternative timing references when normal gravitational information is unavailable. These approaches were not tested in the present study and will require separate evaluation.
Important limitations
The experiment involved only 12 participants, limiting how broadly its results can be generalised. Most had previous parabolic-flight experience, and people considered highly susceptible to motion sickness were excluded.
All participants completed the microgravity condition before the partial-gravity conditions, meaning the researchers could not completely separate gravity effects from possible order or learning effects. The tasks were also self-paced, so movement frequency changed between conditions and may have interacted with gravity.
Parabolic flights provide only brief periods of altered gravity. The gravity phases in this experiment lasted approximately 20 to 50 seconds, with only the central 10 seconds used for comparison. These short exposures cannot show how the motor system might adapt during days, weeks or months in space.
Continuous visual feedback was provided throughout the tasks. Coordination performance without this feedback, or with auditory and tactile cues, could produce different results.
Gravity as part of the human movement system
The study provides evidence that gravity is an active part of the environment in which human coordination develops and operates. In-phase movement remained relatively stable across conditions, but more demanding patterns became increasingly vulnerable when gravitational input disappeared.
Partial gravity helped restore coordination, supporting the view that gravity supplies both physical loading and sensory structure. Understanding that relationship could become increasingly important as astronauts perform longer and more complex activities across microgravity, lunar gravity and Martian gravity environments.